This invention provides an in-press method for applying a painted surface to a piece of hardboard or fiberboard while the board is being formed in a press.
Over 20 billion square feet of wood hardboard and fiberboard are produced annually. Most of this footage goes into house and furniture construction such as sheathing, acoustical tile, siding, paneling, underlayment, dustproofing, drawer bottoms, case backs and doors. Hardboard and fiberboard can be produced by thermomechanical pulping and hot press densification techniques first developed by William Mason in 1925.
Hardboard and fiberboard are generally manufactured from exploded wood fibers which are loosely stacked as a layer in a press and compacted under heat and pressure to form a relatively dense and rigid board. Raw materials for the board can come from whole wood, sawdust and mill residues, waste paper, agricultural wastes, and plant tissue other than woody stems. Most hardboard and fiberboard manufactured today is produced by thermomechanical pulping in which the raw material is placed in a cylinder under heat and pressure and exploded through an opening similar to a constrictive nozzle to fragment the material into a mixture of fiber, fiber bundles, and hard shives. The exploded wood fibers are placed in a heated press, and the press platens apply pressure of approximately 500 to 750 psi and heat in excess of 400.degree. F. to compress the fibrous mass into a relatively rigid board. The thickness of the original layer of loose wood fibers can be reduced up to about 16 times its original size during the pressing operation.
There are distinctions between "hardboard" and "fiberboard" as they are known in the industry. Hardboard is a high density board which can be embossed and machined, and the product sold under the trademark Masonite is typical. Hardboard is formed from exploded wood fibers essentially without any added binders or other additives. The high lignin content of the wood fibers fuses the fibers together when heat and pressure are applied in the press.
Fiberboard is a somewhat medium density board which is also made from exploded wood fibers, except that a small amount, say about 5% by weight, of added binders or other additives are included in the raw material which goes into the press.
The present invention can be used for both hardboard and fiberboard, although the description to follow will refer only to hardboard for simplicity.
After the hardboard is formed in the press, a finished surface can be applied by passing the hardboard to a separate finishing plant where the surfaces are sanded or polished in preparation for applying decorative and protective paint layers or laminae.
The finishing operation is expensive and time-consuming. The cost of a second press for applying decorative laminae can greatly increase the cost of the finished product as well as requiring the additional time to finish the product.
A separate painting operation requires a long production line having spray equipment, or the like, for applying the paint, ovens for drying the paint, and equipment for exhausting paint vapors from the working areas of the plant. In addition, control measures are required for keeping dust particles away from the painted surfaces and to avoid runoff on high spots of the finished board. The cost of the separate production line for applying painted surfaces can greatly increase the cost of the finished board as well as adding to the time required to produce it.
This invention provides a method for forming hardboard and simultaneously applying a painted surface to the board while it is being formed in the press. Thus, secondary finishing processes for applying decorative laminae or painted surfaces are eliminated.